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Help needed to upgrade the capacitor in OTL tube amp - Page 6

post #76 of 113
Originally Posted by telecaster View Post

I haven't found 22K in my stach, could I use 25K without detrimental effect? It is very interesting thanks for the knowledge WALL-E!


I received the BC 1000µF 200V and installed them. I added 30µF MKP caps to the output coupling, and I removed all the bypass caps too. I think it must be the brand new caps but I find the sound lacking bass. The sound is relax though but the bass quality is not there. I will let the amp burn couple dozen of hours and see if it improves.

Overall I prefer the sound before this mod, highs were more calm, and bass were deeper stronger. i strongly suspect the caps needing burn in.


With the supply 90 VDC, 25K Ohm Bleed Resistor and  230uF(summary) output capacitor u will need wait about 6 second to connect headphones without poping noise. If after that u will hear single crack/popup noise then lower the valu of bleed resistor.

Like you said, many parts has been changed it is now like a new amp and needing burn in, to get it best!

post #77 of 113
Thread Starter 

I can wait 7 secondes no problem! I am already waiting somewhere around 10 secondes to plug my HP everytime... The thing is I found caddock MP930 in 25K so thats what I'll use.


The sound is definately better after 15 hours burn! I didn't believed it but more and more I begin to think this is not psychological, the brand new cap need some votage through them that is for sure! The sound was very nice without the bypass caps.


I will try with the bypass caps now and report later after some burn in.


Edited by telecaster - 6/19/12 at 2:09am
post #78 of 113
Thread Starter 

I recieve a pair of L63 (6J5) tube and I wanna try it! On the darkvoice 339 thread, Yuking said that I don't need to change any parts only the tube wiring.. But in Keph modded 339 that uses 6J5, there is a 100R between the 220µF caps while stock 339 has 200R value there.

Do you know what specs of the L63/6J5 tubes I have to look for to know if 200R is safe?

post #79 of 113
Thread Starter 

I guess 200ohms holds more voltage and current than 100ohms so I went ahead! I did the 6J5 mod! I am very happy because the bass is really better with those osram driver.

post #80 of 113

In the stock LF339 the last two resistors in series in ps filter are 200 Ohm and 30K Ohm and if u look closer on keph modded machine they are100 Ohm and 33K ohm. The input penthode 6js7,6j7gt..etc in the 339 is triode-strapped connected (grounded plate and screen as plate configuration) with the G2 as the output of a pentode that's mean pentode is used in triode mode.That's why it possible to use "real " Triode(6j5) as a driver instead of 6js7(pentode).

back to the two resistors in ps filter in orginal unmoded 339 they are 200ohm in series with 30K ohm so after B+ rail for 6js7 the plate voltage is about 73V(mesured with 6as7&6js7gt in ) and  It's drawing about 2.19mA of plate current at idle, which means it is ok. according to the data sheets on the 6SJ7 shows typical operation at 2-3ma plate current.
Characteristics and typical operationwith point of the 6j5 (Medium-Mu Triode) is ideal when running them at higher currents at lest ( 8 > or = 8mA ) It tends to run them at 8 - 10mA typically.
I think these two resistors(100 Ohm & 33k) in keph's modded lf339 were chosen to adjust current and voltage swing to get the optimal operation point and linear characteristics with the 6j5.



I hope the LF boss will join to the discussion and explain it normal_smile%20.gif better.

Edited by WALL-E - 6/20/12 at 4:28pm
post #81 of 113
Thread Starter 

Yes I hope too! But it seems the language is a barrier here!

I bought the same 33K caddock the boss is using but I am using 200ohm resistor for B+ rail right now because I didn't receive the 100ohm resistor yet. Sound is great but I have some hum/noise which I don't know if it's the tube itself or the problem in current.. I will report back when 100ohm arrives.


I have an idea to combat a problem that may or not apply here. I read that someone cured buzzing by filtering the AC filament heater supply 6,3V with capacitor (10000µF/16V) and a bridge to produce DC 6,3V. In his case he has two AC 6,3V supply, one for the power tubes, and one for the drivers. He only filtered the driver AC 6,3V and said it was much more cleaner. He said due to huge current draw - the 6,3AC was very distorted before  (individual secondary had to supply almost 4A of current) so the buzzing had many harmonics above 50 Hz.


I guess this doesn't apply to LF339 because there is only one AC 6,3V supply right?

post #82 of 113

The LF339 use the common 6.3 volt AC / 3A secondary to heat the filament, but it doesn't mean it can't be DC apply to this amp, it is still possible! The rca 6as7,6080..drawing about 2.5A of filament current plus about 300mA for a 6sj7,6j5..etc. it is a lot but power tranny can handle it.

let's see.

Convert 6.3V AC onto DC to heat all the filaments is bad idea because the high current, but if you split the common 6.3V and let the power section be AC powered and for driver tube u went onto DC using a couple of low forward voltage drop diode(Schottky) and a couple of thousands(uF) caps with a dropping resistors it could work.


upright heater2.jpg


Another option more expensive is to buy separate or custom wound transformer with the 6.3V and 9V AC secondary to heat the filaments.
To obtain the perfect extra smoothing 6.3VDC u need about 9VAC secondary to use with low drop out regulator like LM317,LM350...etc.  


AC voltage can generate an AC electric field, which affects the grid, introducing noise that gets into everything, but DC magnetic field source is the DC current! so?
It doesn't mean you cannot use DC, but to keep AC hum low can be achieved by using standard practices like:
1. Proper lead dress, and twisting of heater pairs, twisting the heater wires greatly reduces the magnetic field that is radiated.
2. It is also a good idea to use a shielded filament transformer, which lf339 it already has.
3. Shielding heater leads is only to reduce the risk of RF contamination but can be apply.
4. Another tactics to suppress the noise on AC heaters is using small ceramic caps ( typically use a 0.1uf) from each heater pin to earth is a good idea.

It seems that symmetrical layout of wires in lf339 work well for me but if not (during the further upgrade/mod) I will rewire the filament wires twisting the cables.

So for now I will follow rule number one: "if it works, don't touch it".

Edited by WALL-E - 6/23/12 at 11:10am
post #83 of 113
Thread Starter 

This is really interesting, I thank you for the sound advices and most likely if I build a new amp from scratch will follow thoses rules in making quiet driver PS. As for the 339, this is only one pair of tubes that hums slightly, nowhere near as my old 6SJ7 so it is really acceptable all in all! Also when changing to 470µF the C4 and C5 I noticed the noise floor lowered significantly and also the bass and control is better than with 220µF value.

You are right for now the layout work well so I think I won't mess with it.

post #84 of 113
Thread Starter 

@Wall-E : With all the left over parts I want to make another headphone amp. It seems I just need two transformer and tube sockets. Do you know what transformer should I buy? I found on ebay cheap transformer with 220V primary and 6V secondary rated 30VA...

Or maybe it must be better to find good quality transformer to replace those in the 339?

Will you be using the stock transfomer in you mod? I think Yuking is upgrading the transformer in his mods.

post #85 of 113

Why not build something totally different?


Maybe its just me, but I can never stand to build 2 of the same. For the crazy-money one could spend on "better" parts to build another of the same in the hopes of finding marginal differences one could just build something different. Like a SET amp. Or a parafeed amp. Or a Push-pull amp. Or a hybrid. Or an SS amp with transformer coupled outputs. Or any number of things that ARE different. 

post #86 of 113
Thread Starter 

You are completely right! I just am totally new to valve circuits, so replicating the other amp was the only straight forward thing I was thinking of.

The thing is I have loads of 6AS7G and 6SJ7 tubes, the later being unused now that I transform my amp to 6J5 driver tube.

Also I wasn't going for an exact same replicate, I wanted a 2000µF output capacitor coupled amp to drive low impedance cans.


Maybe you have some idea to share about a louspeaker amp using 6AS7G and 6SJ7? Also I don't think OTL will I be able to use commercial loudspeaker with...

Also the capacitor I have on hands are only rated 200V...

Edited by telecaster - 6/26/12 at 8:19am
post #87 of 113
Thread Starter 

I am thinking of making a converter to use 6SJ7GT with 6J5 socket. WALL-E you said plate current is 3ma for 6SJ7 but in this specs sheet http://www.r-type.org/pdfs/6sj7.pdf it is stated that in triode connection mode the plate current is 6ma to 9ma...

I wonder if doing a converter to use 6SJ7 is worth it, I just have sockets on hands, and as I am rewiring the EF80 converter Yuking sold me into EF86 converter (I will post the pinouts for reference later).

post #88 of 113



Updated: full schematic with my measurements

Edited by WALL-E - 8/10/12 at 2:09pm
post #89 of 113
I was quite busy for the last to months"sumer hollyday", but now back to reality. With an empty wallet, but with big intentions preparing to "mod" the amp soon, ordered most of parts, needed to order output coupling capacitors, not decide yet which one because lack of space u know for one large single capacitor for each channel.
Telecaster u used 4 solen 100uf/250 in place of stock cap I think to use Mundorf MKP they have similar dimensions to Solen, did you have any difficulties to fit them and put the lid back?

Edited by WALL-E - 8/10/12 at 2:11pm
post #90 of 113
Thread Starter 

I choosed the 100uF because the retailer specs stated size within the inner size of the lid. When all finished there is no problem to fit the four caps, the heatshink over the solder ends. In reality the caps are 2mm smaller than the specs and I think I could have fitted 120uF caps for sure, and maybe even 133uf.

My choice was just to let the lid off because it looks salty to me hehe!

Thanks for the schematics, good works! It looks very neat and fully comprehensive. I have found great impact in bass and distorsion tuning when changing the value of R4. In my modded 339, 33K was very clean but little sterile in the bass. 10K was too much bass and very distored, and I finally use 25K with great bass and good tube bloom. I want to try 22K one day just for the fun. In my limited measurements, raising R4 lowered current on V2 and increased current on V1. 

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